Researchers led by the University of Tsukuba present an improved way to model interactions between matter and light at the atomic scale

Tsukuba, Japan—Light-matter interactions form the basis of many important technologies, including lasers, light-emitting diodes (LEDs), and atomic clocks. However, usual computational approaches for modeling such interactions have limited usefulness and capability. Now, researchers from Japan have developed a technique that overcomes these limitations.

In a study published this month in The International Journal of High Performance Computing Applications, a research team led by the University of Tsukuba describes a highly efficient method for simulating light-matter interactions at the atomic scale.

What makes these interactions so difficult to simulate? One reason is that phenomena associated with the interactions encompass many areas of physics, involving both the propagation of light waves and the dynamics of electrons and ions in matter. Another reason is that such phenomena can cover a wide range of length and time scales.

Given the multiphysics and multiscale nature of the problem, light-matter interactions are typically modeled using two separate computational methods. The first is electromagnetic analysis, whereby the electromagnetic fields of the light are studied; the second is a quantum-mechanical calculation of the optical properties of the matter. But these methods assume that the electromagnetic fields are weak and that there is a difference in the length scale.

“Our approach provides a unified and improved way to simulate light-matter interactions,” says senior author of the study Professor Kazuhiro Yabana. “We achieve this feat by simultaneously solving three key physics equations: the Maxwell equation for the electromagnetic fields, the time-dependent Kohn-Sham equation for the electrons, and the Newton equation for the ions.”

The researchers implemented the method in their in-house software SALMON (Scalable Ab initio Light-Matter simulator for Optics and Nanoscience), and they thoroughly optimized the simulation computer code to maximize its performance. They then tested the code by modeling light-matter interactions in a thin film of amorphous silicon dioxide, composed of more than 10,000 atoms. This simulation was carried out using almost 28,000 nodes of the fastest supercomputer in the world, Fugaku, at the RIKEN Center for Computational Science in Kobe, Japan.

“We found that our code is extremely efficient, achieving the goal of one second per time step of the calculation that is needed for practical applications,” says Professor Yabana. “The performance is close to its maximum possible value, set by the bandwidth of the computer memory, and the code has the desirable property of excellent weak scalability.”

Although the team simulated light-matter interactions in a thin film in this work, their approach could be used to explore many phenomena in nanoscale optics and photonics.

###
This research was supported by MEXT as a priority issue (theme 7) to be tackled using the Post-K Computer; by JST-CREST (grant number JP-MJCR16N5); and by the MEXT Quantum Leap Flagship Program (MEXT Q-LEAP, grant number JPMXS0118068681).

Original Paper

The article, “Large-scale ab initio simulation of light-matter interaction at the atomic scale in Fugaku,” was published in The International Journal of High Performance Computing Applications at DOI: https://doi.org/10.1177/10943420211065723

Correspondence

Professor YABANA Kazuhiro
Center for Computational Sciences, University of Tsukuba

Researchers led by the University of Tsukuba present computer simulations that capture the complex dynamics of elusive neutrinos left over from the Big Bang

Tsukuba, Japan – Current simulations of cosmic structure formation do not accurately reproduce the properties of ghost-like particles called neutrinos that have been present in the Universe since its beginning. But now, a research team from Japan has devised an approach that solves this problem.

In a study published this month in SC ’21: Proceedings of the International Conference for High Performance Computing, Networking, Storage and Analysis, researchers at the University of Tsukuba, Kyoto University, and the University of Tokyo report simulations that precisely follow the dynamics of such cosmic relic neutrinos. This study was selected as a finalist for the 2021 ACM Gordon Bell Prize, which recognizes outstanding achievement in high-performance computing.

Neutrinos are much lighter than all other known particles, but their exact mass remains a mystery. Measuring this mass could help scientists develop theories that go beyond the standard model of particle physics and test explanations for how the Universe evolved. One promising way to pin down this mass is to study the impact of cosmic relic neutrinos on large-scale structure formation using simulations and compare the results with observations. But these simulations need to be extremely accurate.

“Standard simulations use techniques known as particle-based N-body methods, which have two main drawbacks when it comes to massive neutrinos,” explains Dr. Naoki Yoshida, Principal Investigator at the Kavli Institute for the Physics and Mathematics of the Universe, the University of Tokyo. “First, the simulation results are susceptible to random fluctuations called shot noise. And second, these particle-based methods cannot accurately reproduce collisionless damping—a key process in which fast-moving neutrinos suppress the growth of structure in the Universe.”

To avoid these issues, the researchers followed the dynamics of the massive neutrinos by directly solving a central equation in plasma physics known as the Vlasov equation. Unlike previous studies, they solved this equation in full six-dimensional phase space, which means that all six dimensions associated with space and velocity were considered. The team coupled this Vlasov simulation with a particle-based N-body simulation of cold dark matter—the main component of matter in the Universe. They performed their hybrid simulations on the supercomputer Fugaku at the RIKEN Center for Computational Science.

“Our largest simulation self-consistently combines the Vlasov simulation on 400 trillion grids with 330 billion-body calculations, and it accurately reproduces the complex dynamics of cosmic neutrinos,” says lead author of the study, Professor Koji Yoshikawa. “Moreover, the time-to-solution for our simulation is substantially shorter than that for the largest N-body simulations, and the performance scales extremely well with up to 147,456 nodes (7 million CPU cores) on Fugaku.”

In addition to helping determine the neutrino mass, the researchers suggest that their scheme could be used to study, for example, phenomena involving electrostatic and magnetized plasma and self-gravitating systems.

Original Paper

The article, “A 400 trillion-grid Vlasov Simulation on Fugaku Supercomputer: Large-scale Distribution of Cosmic Relic Neutrinos in a Six-dimensional Phase Space,” was published in SC ’21: Proceedings of the International Conference for High Performance Computing, Networking, Storage and Analysis at DOI: 10.1145/3458817.3487401

Funding and acknowledgements

This research is supported by MEXT as “Priority Issue on postK computer” (Elucidation of the Fundamental Laws and Evolution of the Universe) and “Program for Promoting Researches on the Supercomputer Fugaku” (Toward a unified view of the universe: from large scale structures to planets). This research is also supported by the JSPS KAKENHI Grant Number JP18H04336 and JP21H01079, by JST CREST JPMJCR1414 and by JST AIP Acceleration Research Grant JP20317829. Our code has been developed partially on ATERUI supercomputer at Center for Computational Astrophysics (CfCA), National Astronomical Observatory of Japan in its early stage. This research also used computational resources of the Oakforest–PACS through the HPCI System Research Project (project ID: hp170123 and hp190093) and Multidisciplinary Cooperative Research Program in Center for Computational Sciences, University of Tsukuba (project ID: 17a40 and xg18i019).

Correspondence

Associate Professor YOSHIKAWA Kohji
Center for Computational Sciences, University of Tsukuba